Just discovered this bug on my hunter alt, Concussive, over on Benediction - while questing in Mount Hyjal. In the Shrine of Assessnia area.
The vendor npcs associated with the Traveler’s Mammoth mount seems to not want to spawn in, but as soon as I leave Mount Hyjal… To, say, Dalaran - the vendor NPCs spawn in as expected.
Hopefully this is just a temporary bug or something related to quest phasing causing the npcs to not spawn in.